1. Field of the Invention
The present invention relates to an optical device to be used for transmitting light and a fabrication method and an apparatus for such an optical device.
2. Description of a Related Art
FIGS. 17A-17C are diagrams for explaining a conventional fabrication method for fiber collimators. An end surface 901a of an aspheric lens 901 shown in FIG. 17A is flat-polished and coated with an anti-reflective film (AR coat) in order to reduce a reflection loss at a wavelength of light to be transmitted. On the other hand, an optical fiber 902 is inserted into a sleeve called as a capillary 903, which is precisely drilled, and fixed by an adhesive 904 such as resin. An end portion of the optical fiber 902 is cut together with the capillary 903, and an end surface 902a thereof is flat-polished and AR-coated.
When splicing these two devices 901 and 902, as shown in FIG. 17B, an adhesive 905 such as resin is arranged between the end surface 901a of the lens and the end surface 902a of the optical fiber, and optical axis alignment is performed by using a highly reflective mirror 906. That is, the position of the optical fiber 902 on the X-Y plane relative to the lens 901 is adjusted such that light, which is caused to enter the optical fiber 902 and reflected by the highly reflective mirror 906 via the lens 901, returns again to the optical fiber 902 via the lens 901 most efficiently. In such an arrangement, by curing the adhesive 905, a fiber collimator shown in FIG. 17C is completed.
As described above, in the conventional fabrication method for fiber collimators, the number of devices to be used is large and processes required for fabrication are also complicated. Further, since it takes such a long time as several minutes to 30 minutes to cure the adhesive, time for assembly is lengthened and the cost is increased. Furthermore, since a resin adhesive is interposed between the lens and the optical fiber, an optical problem of distortion etc. of a plane of polarization, a problem of deviation of an optical axis that might be caused until the adhesive is cured, and a long-term problem of deterioration of the adhesive and so on arise. Still further, in a lens used in such a fabrication method, a focus F thereof is normally set on the end surface 901a of the lens, and therefore, when light having high power is caused to enter an optical fiber, the light from the lens converges on a minute region at which the adhesive 905 is arranged, resulting in a problem that the spliced portion is easily damaged.
As an optical device not using a resin adhesive, Japanese Patent No. 2876857 (p. 1, FIG. 1) discloses an optical waveguide device in which a fused material, which has a softening temperature lower than that of the material of both the end surface of the optical waveguide and the end surface of the optical fiber, is interposed in the connection plane only within a range of the plane shared by the connection portions of the both end surfaces, and the fused material and the both end surfaces are connected integrally by fusing within the range of the above-mentioned shared plane. In such an optical waveguide device, although the problem caused by interposing a resin adhesive can be avoided, a third member (fused material) is used in addition to the optical waveguide and the optical fiber, and therefore, the fabrication process thereof is also complicated.
Therefore, a technology capable of fabricating an optical device without interposing an adhesive or a fused material is proposed. Ide et al., “A Novel Fabrication Method for Fibre Collimators Using “Shrink-Fit” Splice by Arc Discharge Heating”, European Conference on Optical Communication (Proc. ECOC2004), We4.P.020 (2004), discloses a novel fabrication method for fiber collimators, which is capable of directly splicing a single mode fiber and a multi-component glass base lens. Further, Takahara, “Assembly Technology for Ultra Microlens Array”, the 92nd microoptics and the 6th system photonics joint meeting (July, 2004), discloses a future prospect of microoptical devices using such a novel fabrication method.
The fabrication method for fiber collimators disclosed in the above-mentioned documents by Ide and Takahara will be explained with reference to FIGS. 18A-18C. In the fabrication method for fiber collimators, a low melting point glass lens and a quartz optical fiber are used as devices.
First, as shown in FIG. 18A, optical axis alignment of a core 912b of an optical fiber 912 is performed with respect to a lens 911. To this end, a highly reflective mirror 913 is arranged ahead of the lens 911 (on the left-hand side in the drawing) and perpendicular to the optical axis of the lens 911, and at the same time, an end surface 912a of the optical fiber 912 is brought about 5 μm close to a rear end surface (on the right-hand side in the drawing) 911a of the lens 911. Then, light is caused to enter the optical fiber 912 and the intensity of the returned light is detected, which light has been reflected by the highly reflective mirror 913 via the lens 911 and has again entered the optical fiber 912 via the lens 911. While monitoring the intensity of the returned light, the end surface 912a of the optical fiber 912 is moved in the X-Y plane and the position of the end surface 912a of the optical fiber 912 is determined such that the returned light is strongest.
Next, as shown in FIG. 18B, an arc electrode 914 is arranged in the vicinity of the end surface 911a of the lens 911 and by causing an arc discharge to occur to generate a thermal plasma, the end portion of the lens 911 including the rear end surface 911a is softened. Then, as shown in FIG. 18C, the optical fiber 912 is pushed in to a focus F present at a depth of about 5 μm to 20 μm from the end surface 911a of the softened lens 911. The optical fiber 912 is fixed by keeping this arrangement and terminating the arc discharge to allow the lens 911 to cool down spontaneously.
According to such a fabrication method for fiber collimators, no capillary is necessary and processes such as polishing of the surface to be spliced and formation of an AR coat are not necessary, and therefore, the cost can be reduced. Further, there is an advantage that the assembly time can be reduced to about a few seconds because fused-splicing is performed by using arc discharges. Furthermore, since an adhesive such as resin is not used, it is possible to avoid problems such as distortion of a plane of polarization caused by adhesive, deviation in position between a lens and an optical fiber, and deterioration of adhesive and damages to adhesive when light having high power enters in a long term.
However, there arises another problem as follows in a fabrication method for fiber collimators by using the fused-splicing.
Firstly, as shown in FIG. 19, if spontaneously cooling down is allowed in the arrangement in which the optical fiber 912 is pushed into the softened lens 911, a compression stress is applied to the portion into which the optical fiber 912 is pushed due to the contraction of the glass. Accordingly, crack is introduced in the optical fiber 912 and the optical fiber breaks at its root.
Secondly, in a lens to be used in such a fabrication method, the focus F is normally positioned inside the lens 911 in which the end surface 912a of the optical fiber 912 is arranged. In other words, as shown in FIG. 18A, on the X-Y plane on which the end surface 912a of the optical fiber 912 is arranged at the time of optical axis alignment, the focus of the lens 911 does not exist. Because of this, optical axis alignment is performed based on the returned light in a blurred state (that is, an arrangement in which the center peak of the intensity is broadened), and therefore, the precision in detecting the center of the X-axis and Y-axis is deteriorated. Further, since it is not possible to confirm the position of the focus F existing inside the lens before fused-splicing, it is difficult to precisely match the end surface 912a to the focus F when pushing the optical fiber 912 into the lens 911. Therefore, the precision about Z-axis is also deteriorated.
Thirdly, as shown in FIG. 20A, in the case of fabricating a fiber collimator by employing an optical fiber having a front end which is cut obliquely, when an optical fiber 922 is pushed into a lens 921, an inclined end surface 922a of the optical fiber 922 receives a reaction force in the direction perpendicular to the end surface 922a from the softened glass. As a result, as shown in FIG. 20B, the front end portion of the fiber 922 is fixed at a deferent position deviated from the position at which optical axis alignment is performed. Therefore, there also arises a problem that optical devices having high precision cannot be fabricated.